Pine cones only open when the weather is dry and warm; feather grass releases its seeds when the air humidity is high. Nature is full of environmentally adaptive materials and has since inspired many man made materials. In the field of building engineering for example, temperature and humidity dependent actuators are the key to intelligent facades and roofs that can respond to changing weather conditions.

Two key parameters –temperature and humidity – play a key role in modulating stimuli-responsive commodity materials. Until now, the simultaneous interaction of both temperature and humidity on adaptive structures has been less explored. Recently Verpaalen et al. have generated dual-responsive heat/humidity bilayer actuators and examined the dual-responsivity to changes in humidity and temperature.

In this work, the researchers fabricated the dual-responsive bilayer actuators by combining a humidity sensitive polyamide 6 (PA6) layer and a temperature sensitive liquid crystal network (LCN) coating. Self-alignment of the LCN is verified by polarised infrared spectroscopy. The pre-bent shape of the bilayer actuators was observed because the order of the LCN layer increased and expanded along the alignment direction due to the cooling process during fabrication.

The dual-responsivity towards humidity and/or temperature changes was demonstrated by submitting the material to a well-defined temperature and humidity. The bilayers showed different bending behaviour. At low relative humidity (5% RH), the bending angle was constantly around -50° even as temperature increased from 25 °C to 83 °C (red dots).

But at high relative humidity (80% RH), the bending angle dramatically decreased from +50° to -50° as temperature increased from 28 °C to 62 °C (red circles). At constant temperature (20 °C), the bending angle increased from -50° to +75° as the relative humidity increased from 7% to 80% (blue dots).

This behaviour can be explained by the fact that PA6 includes amorphous and crystalline blocks with polymer chains interacting through intermolecular hydrogen bonds, and free amide groups that can ab- and desorb water molecules. At low humidity, as temperature increased above T_g(LCN) ∼49 °C, the LCN undergoes a transition from a glassy to a rubberlike material. Less conformational restrictions drive the expansion or contraction of the LCN layer. However, the LCN on the outer diameter of the curvature failed to bend the much thicker PA6 layer (red dots). At high humidity, the PA6 layer bound significant amounts of moisture—further softening and expanding or contracting itself.

Humidity generator and controller
The humidity generators of the HGC series allow for a reliable control of the relative humidity, in the range of 5% to 90% (between 5 °C and 85 °C), inside small environmental measuring chambers from DataPhysics Instruments or other suppliers. They are able to generate dry air without an external pressurized gas supply through an internal self-regenerating desiccant reservoir. In combination with the integrated water heating system precise control of the moisture level is possible. Due to the integrated touch screen the system can be operated without additional software and is directly ready for use.

The authors have managed to designed dual-responsive bilayer actuators sensitive towards humidity and temperature changes. They demonstrated different bending behaviours of PA6/LCN bilayers by controlling the temperature and humidity changes in the environment. PA6/LCN bilayers are expected to have a tremendous impact on current environmentally adaptive materials fabrication and hold potential promise for various applications.

A humidity generator and controller HGC 30 was used in this research. For more information please refer to the original publication: